The invention relates to a liquid distributor for a mass-transfer column and to a corresponding method, the liquid distributor having at least one main channel and a multiplicity of distributing channels which are in flow connection with the main channel and are provided with liquid-outlet openings, of which the effective length L is smaller than their diameter D, it being the case that the effective length L is established by that part of the boundary of the outlet opening which is wetted by the liquid during operation of the distributor, and the diameter D denotes the maximum linear extent of the free cross section of the liquid-outlet opening.
Distributors of this type serve for distributing liquid over the cross section of a mass-transfer column, in particular above a section provided with ordered or random packing. They are known, for example, from an article by P. Bomio et al, Chem. Tech. 43rd year, Issue November/December 1991 and are referred to therein as pipe distributors, element distributors or channel distributors. These types of liquid distributors have horizontally aligned distributing channels which are fed by at least one main channel, which usually runs perpendicularly with respect to the distributing channels. In the case of a pipe distributor, the distributing channels are closed on their top side, and in the case of an element or channel distributor they are open at the top. This leads to the problem where the flow in the longitudinal direction of the distributing channels causes a non-uniform distribution of liquid. Depending on the distance between a liquid-outlet opening and the liquid inlet into the distributing channels, said opening has the liquid flowing through it at different outlet speeds.
The object of the invention is therefore to develop a distributor, and a method, of the type mentioned above which allows a particularly high quality of distribution and requires just relatively low production outlay.
As far as equipment is concerned, this object is achieved in that the hole cross section D of the liquid-outlet openings on the inside of the distributing channels differs from the hole cross section E on the outside of the distributing channels.
As far as the method of distributing liquid in a mass-transfer column by means of a liquid distributor is concerned, it being the case that the liquid flows into a multiplicity of distributing channels via a main channel and passes out of liquid-outlet openings located in the distributing channels, it being the case that the liquid wets the liquid-outlet opening over a length which is smaller than the diameter D of the liquid-outlet opening, and it being the case that the diameter D denotes the maximum linear extent of the free cross section of the liquid-outlet opening, this object is achieved in that the liquid wets the liquid-outlet opening over its entire circumference, but not over its entire length.
Diameter here is understood as being the maximum linear extent of the free hole cross section; in the generally customary case of circular liquid-outlet openings, this diameter is the diameter of the circle at the narrowest point of the liquid-outlet opening. The liquid-outlet openings are generally arranged in a horizontally aligned wall section of the distributing channels, with the result that they have the liquid flowing through them essentially in a vertical flow direction.
The effective hole length is established by that part of the hole boundary which is wetted by the liquid during operation of the distributor. For example, in the case of a liquid-outlet opening whose cross section tapers upwards, the effective hole length is smaller than the geometric length. Liquid-outlet openings with this property are also referred to as orifice-plate holes hereinbelow. In the tests carried out within the context of the invention, it has surprisingly been found that the use of such orifice-plate holes results in a considerably improved quality of distribution.
The ratio L/D between the effective length and hole diameter of the liquid-outlet openings is smaller than 1, preferably smaller than or equal to 0.9, smaller than or equal to 0.8, smaller than or equal to 0.7 or smaller than or equal to 0.6.
The effective length of the liquid-outlet openings is preferably smaller than the geometric length thereof. The hole geometry is selected such that the hole length which is wetted by the liquid during operation is smaller than the wall thickness of the distributing channels.
The effective hole length, which is reduced in relation to the geometric hole length, is advantageously achieved in that the hole cross section of the liquid-outlet openings on the inside of the distributing channels is smaller than the hole cross section on the outside of the distributing channels. The liquid-outlet opening may be designed to taper conically from the outside inwards. It is also possible for the cross-sectional surface areas of the hole on the inside and on the outside to be shaped differently, however they are preferably the same, and particularly preferably are both circular. It has been found to be particularly expedient to provide liquid-outlet openings in the form of stepped bores, i.e. one bore running through the entire wall thickness of the distributing channel and a blind hole which is arranged concentrically with respect to said bore and penetrates from the outside just into part of the wall thickness of the distributing channel.
However, it may also be expedient to provide a greater hole cross section of the liquid-outlet openings on the inside of the distributing channels than on the outside of the distributing channels. This variant may provide production-related advantages, in particular, in the case of distributing channels which are open towards the top. The hole geometry is advantageously selected analogously to the variants described above. Stepped bores have been found to be favorable in this configuration as well.
The distributing channels are advantageously closed off at least partially towards the top. So-called pipe distributors in combination with the outlet openings according to the invention are particularly favorable since, even in the case of changes in load, this ensures a high quality of distribution of the liquid. The pipe distributor expediently has a circular or rectangular profile. This means that commercially available pipes or rectangular profiles can be used. Rectangular distributing channels additionally have the advantage that they can easily be fitted on the main channel.
For stability reasons, the wall thickness of the distributing channels is preferably between 1.5 mm and 12 mm, particularly preferably between 2 mm and 6 mm. If, in particular in the case of small hole diameters, the wall thickness exceeds the diameter of the liquid-outlet opening, then the hole geometry is modified such that the effective hole length according to the invention is smaller than the hole diameter.
The diameter of the outlet openings is advantageously from 1 to 10 mm, particularly advantageously from 2 to 6 mm. It has been found that a particularly favorable distribution of the liquid is achieved with this selection of the hole diameter.
From a production point of view, it is favorable to use distributing channels which are produced from extruded profiles or from individual parts which are connected to one another. As a result of their low weight, distributing channels made of plastic or of aluminium are advantageous.
The liquid-outlet openings are preferably arranged in the base of the distributing channels. However, a high quality of distribution is also achieved if the outlet openings are located in the side wall of the distributing channels, particularly advantageously in the bottom third of the side walls.
Moreover, the liquid distributor may have a flow separator which is arranged essentially horizontally within the distributing channels. A flow separator here is understood as being an apparatus by which a medium flowing in one direction (e.g. horizontally) is divided up into two part-streams flowing in different directions (e.g. into a stream which is still horizontal and a stream with a vertical direction component). The flow separator is preferably arranged such that, during operation of the distributor, it is located beneath the surface of the liquid flowing, or static, in the corresponding distributing channel. It causes flow resistance in the vertical direction. This results in two flow cross sections; in the top one the liquid is transported along the distributing channel, and in the bottom one flow takes place merely in the direction of the liquid-outlet opening. The reduction in the horizontal flow speed immediately above the hole brings about a further improvement in the quality of distribution.
The flow separator preferably has a perforated plate. This contains at least one essentially horizontal section. The opening ratio in this region is, for example, 3 to 13%, preferably 5 to 8%, and most preferably approximately 5%.
The invention also relates to a mass-transfer column with a distributor of this type and to the use of such a mass-transfer column for low-temperature separation of air.